Conventionally, in optical instruments, lenses and barrels are made of materials having small temperature dependence so that the focus position does not vary as the temperature changes and the optical instruments are therefore stable with respect to temperature. In some cases, plastic is used as the material of the lenses in order to reduce the cost and to form an aspheric surface. In those cases, it is necessary to decrease the power of the plastic lenses, dispose the plastic lenses in positions at which the plastic lenses are less affected by temperature changes, or use a plurality of plastic lenses to cancel the influence of temperature changes.
Moreover, for example, a lens for a projector is subjected to focus adjustment immediately after the apparatus is turned on, and after that, focus adjustment is not performed. On the other hand, due to heat from an illumination system within the apparatus, the temperature of the lens increases. Therefore, when focus adjustment is performed at the start of operation and focus adjustment is not performed thereafter, a product especially is required to have temperature characteristics that prevent the focus from shifting with temperature.
Patent Document 1 proposes a temperature-corrected optical device in which variations in the focus position due to temperature changes are corrected. In this temperature-corrected optical device, the optical design is devised so that a change in the barrel length according to the coefficient of linear expansion of the material of the barrel and a change in the lens focus position cancel each other.
Patent Document 2 proposes a photographing device having a temperature compensation function. In this photographing device, a lens system is divided into two parts by two lens holding barrels held by a main barrel, and the coefficient of linear expansion of the main barrel is larger than the coefficients of linear expansion of the lens holding barrels. With this configuration, variations in the focus position due to temperature changes are reduced by changing the distances between the lenses.
Patent Document 3 proposes a projection television lens assembly having a temperature correction mechanism. In this projection television lens assembly, a bar member is used to change the distance between portions of the optical system in response to temperature changes so that the temperature changes are accounted for, thereby preventing variations in the focus position.
Patent Document 4 proposes a zoom lens barrel having a temperature correction mechanism. In this configuration, a cam ring constituting the zoom barrel is coupled via a correcting tube made of a material having a coefficient of linear expansion different from that of the cam ring. As a result, as the temperature changes, all of the lens elements integrally move along the optical axis, and thus a temperature correction is achieved. Moreover, the distances between lens groups can be changed by simultaneously rotating the cam ring and the correcting tube, so that the function of a zoom lens can be provided.
However, the temperature-corrected optical device described in Patent Document 1 is effective for a simple optical system such as a collimator, but regarding a lens that requires a long back focal length and for which chromatic aberration is to be corrected at a high level, the flexibility in lens design is insufficient, and it is difficult to design the lens.
Moreover, according to the photographing device described in Patent Document 2, the distance between the two parts, into which the optical system is divided, changes. Therefore, it is necessary to perform optical design so as to prevent variations in aberration. For this reason, regarding a lens that requires a long back focal length and for which chromatic aberration is to be corrected at a high level, the flexibility in lens design is insufficient, and it is difficult to design the lens.
Moreover, according to the projection television lens assembly described in Patent Document 3, the bar member determines the position of a portion of the optical system. Therefore, it is difficult to limit the inclination of the optical system to an allowable value. Since the distance between the portions of the optical system changes, it is necessary to perform optical design so as to prevent variations in aberration. Thus, regarding a lens that requires a long back focal length and for which chromatic aberration is to be corrected at a high level, the flexibility in lens design is insufficient, and it is difficult to design the lens.
Accordingly, Patent Documents 1 to 3 do not propose a temperature correction method that is effective for a zoom lens in which a plurality of lens groups move along the optical axis.
On the other hand, the configuration of Patent Document 4 is a temperature correction mechanism for a zoom lens. However, in the configuration of Patent Document 4, the correction amount is constant regardless of the zoom position. Thus, when a correction amount at a given zoom position is determined, the correction amount becomes excessive or deficient at another zoom position.
More specifically, variations in the focus position of a lens due to temperature changes are caused by changes in the refractive indices of lens elements constituting the lens or changes in the shapes of the lens elements. However, in a zoom lens in which the focal length is changed by changing the distance between the lens elements on the optical axis, the amount of change of the focus position when the temperature changes by 1° C. varies depending on the zoom position (e.g., a wide-angle position or a telephoto position).
For example, when a correction amount is determined on the wide-angle side, as long as the zoom lens is used on the wide-angle side, the focus position remains unchanged even when the temperature changes, so that a high resolution can be maintained. However, when the zoom lens is used on the telephoto side, the focus position changes as the temperature changes, and as a result, the resolution is decreased.    Patent Document 1; JP 06-130267 A    Patent Document 2: JP 06-186466 A    Patent Document 3: JP 2002-544537 A    Patent Document 4: JP 2004-264577 A